1233
V O L U M E 21, NO. 10, O C T O B E R 1 9 4 9 of visual methods of color determination in this laboratory. This phase of the work has been completed. However, further investigations are in progress on the developnient of instruments for measuring the colors of dry white sugars and for photoelectrically indicating the color of refinery liquors on a continuous flow basis. LITERATURE CITED
(6) Keane, J. C., and Brice, B. A . , IND.ENG.CHEM.,ANAL.E D . , 9, 258 (1937).
(12)
Lindfors, K. R., 2nd. Eng. Chem., 17, 1155 (1925). Morse, E. E., and McGinnis, R. -4., I N D . ENG.CHEM.,.INAL. ED., 14, 212 (1942). Nees, A. R., Ibid., 11, 142 (1939). Peters, H. H., and Phelps, F. P., Bur. Standards, Tech. Paper 338 (March 12, 1927). Spencer, G. L., and Meade, 0. P., “Cane Sugar Handbook,’’8th ed., p. 552, New York, John Wiley & Sons, 1945. Zerban, F. W., and Sattler, Louis, IND.ENG.CHEM.,AN.AL.E D . ,
(13)
Zerban, F. W., Sattler, Louis, and Lorge, Irving, Ibid., 6, 178
(7) (8) (9) (10)
(11)
(1) Balch, K.T., I N D .ENQ.CHEU.,ANAL.E D . , 3, 124 (1931). (2) Gillett, T. K.,and Holven, A. L., I n d . Eng. Chem., 28, 391 (1936). (3) Ibid., 35, 210 (1943). (4) Gillett, T. R . , Meads, P. F., and Holven, A. L., U. 6 . Patent 2,356,288 (Aug. 22, 1944). (5) Holven, A. L., and Gillett, T. R., Facts About Sugar, 30, 169 (1935).
9 , 2 2 9 (1937).
(1934). RECLIVED October 4, 1948. Presented before the Division of Sugar Chemistry and Technology at the 114th Meeting of the AXERICAXCHEMICAL SOCIETY,Portland, Ore.
Ammonium Hexanitratocerate as a Primary Standard in Oxidimetry G. FREDERICK SMITH
AND WALTER H. FLY University of Illinois, Urbana, I l l .
Ammonium hexanitratocerate (under the name hexanitrato ammonium cerate, now abandoned) has been described (5) as “a proposed reference standard in oxidimetry.” The present paper presents experimental data in proof of the fact that ammonium hexanitratocerate may now be accepted as an unqualified primary standard reagent. Its use is also standard for the preparation of evaluated Ce(1V) solutions. The process involves a direct weighing, followed by solution in either perchloric or sulfuric acid and dilution to suitable known volumes. A n additional precision process in its use consists in the simple preparation from weighed
A
SUMMARY of work dealing with the use of Ce(1V) in
volumetric analysis complete to 1942 has been provided ( 2 ) . No other Ce(1V) salt has been suggested as a primary standard for use in the preparation of accurately standardized volumetric cerate solutions. Ammonium hexanitratocerate [(NH4)&e(NOa)6]complies with practically all desirable properties of a primary standard chemical. It is a stock item from all standard supply sources in primary standard purity. The reagent grade purity is also manufactured. It is an anhydrous salt stable in air under ordinary conditions and a t 85” C. for extended and adequate periods of time. It has an exceptionally high equivalent weight (548.26) and but one transferred electron is involved in the change from Ce(1V) to Ce(II1). It is instantly and speedily reactive in acid solution under readily attained conditions with solutions of the most reliable primary standards such as arsenious oxide, and sodium oxalate or the less desirable primary standard Mohr’s salt, (NH& S01.FeS01.6Hn0. S o possible intermediate valence stages are involved in its use as an oxidant. A large number of internal redox indicators are applicable in the determination of the precise equivalent quantities of reductants with which it reacts. Ammonium hexanitratocerate is easily tested chemically for purity. Stable standard solutions are readily prepared from it by direct weighing, solution in suitable media, and dilution to definite volume, and are of wide applicability. I n this respect it rivals potassium permanganate, the use of which it is rapidly
quantities of ammonium hexanitratocerate (by precipitation using excess ammonium hydroxide) of ceric hydroxide, Ce(OH),, or hydrous ceric oxide, Ce02.2H~0, in accurately known amounts. The precipitated product thus obtained is easily soluble in warm dilute sulfuric acid. This process makes practical the preparation of standard sulfatoceric acid [H4Ce(S0,)J solutions free from ammonium and nitrate ions whose presence might conceivably be objectionable. It thus serves as a substitute for a corresponding ammonium sulfatocerate with primary standard purity, a reagent that has not yet been prepared.
supplanting in a great many commercially important analytical control and research practices. PREPARATION OF PRIMARY STANDARD AMMONIUM €1EXANITR ATOC ERATE
The preparation of chemically pure ammonium hexanitratoand the same preparational techcerate has been described (6), nique and an alternate procedure were applied in the present work. Two separate samples of the reagent, supplied through the courtesy of The G. Frederick Smith Chemical Company, represented a portion of two 50-pound batches prepared for commercial distribution a t widely separated time intervals to be used as primary standard chemicals. They were analyzed as received without further treatment except for drying a minimum of 1 hour a t 85” C. This makes certain the elimination of any trace of adsorbed moisture. The drying temperature employed would not make certain the removal of any occluded moisture. The results of the analyses, however, made certain the practically complete absence of occluded moisture within the crystal lattice. This obviates the necessity for drying the product a t a higher temperature than that required for the removal of adsorbed moisture. In an alternative preparational procedure dilute nitric acid (1 volume of nitric acid, specific gravity 1.42, diluted to 4 volumes with water) may be substituted for boiling concentrated nitric acid (6) in the purification of ammonium nitratocerate with equally good results.
ANALYTICAL CHEMISTRY
1234 1i ~ l d r1 .
? .
Stability of iinnionium Hexanitratocerate at Moderate Ihying Temperatures
Grama
1
:1.187X
c. 7'1
Ilouin 1
Mg. 10,3 s o 10s.
(i
s o loss
>
8 10
2'
I , W!18
Si
24 2 4 8 8 10
44
s o loss K O loss N o loss 0.5 1.0
s o loss
No loss K O loss 1.2
7c
degree. A 1-gram sainple should be completely soluble in 75 ml. of absolute ethyl alcohol. If 95% alcohol is employed, the test is less valuable and the color of the resulting solution much more intense. This solubility test is mor? stringent than the water-solubility test, because of the greatly diminished soluhility of possible impurities in absolute alcohol.
0 033
0.033 0,033 0,033 0,033 n ,033 0 031 0.062
0.062 0 . 062 0.062
0.074
PRACrIC.4L DETERMINATION O F ( : % S t i 4 L IWPURITIES A MMOh-IUM 11EXA NlTR.4TOC ERATE
IiV
Siinple tests should pIove of value in the preliminary cs:iinination of a supposedly pure sainple of amn~oniumheuanitratocerate. The water-solubility test should show no in*oluhlr matter from a 0.01-mole sample. Dissolve 5.5 grams of the salt i n a 400-ml. beaker, using the least required amount of distilled water a t room temperature, and stirring vigorously to complete solution of the sample. Under these conditions (a practically saturat'ed solution of t.he salt) there is no resultant hydrolysis, as such solutions have a pH of approximately 1. The formation of soluble hydrolytic products with accompanying liberation of free nitric acid (6) accounts for t,his condition. The solut,ion thus prepared should be sparkling clear, red in color, and entirely free from insoluble mat,erial such as ceric oxide. This qualitat,ive test would indicate definitely that the reagent had not been dried a t a higher temperature than that permissible-namely, 85 C. Iron as impurity is the only heavy metal expected. Akpply the iron test to the saturated solution of the sample used in the determination just described. Add dropwise with constant' stirring a 30% solut'ion of hydrogen peroxide until the orange color is almost removed. Complete the reduction of Ce(1V) by the dropwise addition of 3% hydrogen peroxide. Add 500 mg. of hydroxylamine hydrochloride to reduce the iron. Neutralize the resulting solution by the dropwise addition of dilute ammonium hydroxide until Congo red paper changes in color from blue to red. Add 5 ml. of a saturat,ed aqueous solution of 1 10-phenanthroline to complex the ferrous ion and produce the characteristic red color from any iron present ( 4 ) . Carry out an accompanying biank test on the reagents employed, but omit the sample of cerium salt. Dilute bot,h test solutions to 250 nil. and examine for color due to iron. rln excess of 2 p.p.m. of iron as impurity should not be indicated, as shown by comparison with a standard color prepared from a known amount or iron similarly treat'ed. The process of manufacture previousll. described ( 5 ) gives a finished product which by repeated spect'roscopic examination fias been shown to be free from thorium and other ceriumassociated rare metal impurities. These spectroscopic tests have been applied both by the authors and by independent investig:itors a t the University of Chicago. The following analysis is indicative of purity by deteriiiiriatioii of the ignition residue. Place an accurately weighed, approximately 1-gram sample of t,he salt in an ignited and accurately weighed porcelain crucible, and decompose the sample by gradually tieating the cruciblc o v ~ ar very low flame. This process \vi11 leave a residue of ceric oxide which upon subsequent, iguition to constant weight a t 900" C. constitutes 31.396y0 of the original sample. Failure to mettt this test within rcwunable experimental error iridicates a coi,wspondingIy mpure product. A tinal simple solubility test, indicates purity to n mwked
REAGEIVTS EMPLOYED
Ammonium Hexanitratocerate. A G. Frederick Smith (:hemical Company primary standard grade with samples from 50pound batches prepared for distribution t,hrough trade channels. The samples were dried for 4 to 6 hours a t 85" to 87" (.', at. thr time of sampling. Arsenious Oxide. National Bureau of Standards sample 83a. Certificate value, 99.99% purity. Sodium Oxalate. Kational Bureau of Standards saiiiple 4Oe. Certificate value, 99.96% purity. 1,lO-Phenanthroline Ferrous Sulfate, 0.025 dl solution (ferroin). 5-Nitro-1,lO-phenanthrolineFerrous Sulfate, 0.025 ilf solution (nitroferroin). Water. Conductivity water only was employed for thr preparation of all solutions and for all titrations and precipitxtions. Perchloric acid, C.P. vacuum distilled 727%acid manufactured by The G. Frederick Smith Chemical Company. Osmic Acid Catalyst, 0.01 .If solutitrn i n 0.1 A; sulfuric acid. STABILITY OF A R l ~ l O C I D MHEXAVITHATOCER4TE AT \ %RIOlTS 1)RYIYG TEtIPERATCRES
In the original dcwxiptioii of ammonium he\anitr~~tocrrltte as ,i "proposed" primary standard reagent a drying tempprature of 110" C. was spccsified (6) The drying of small samples f o r periods of 1 hour a t this temperature introduces but a minor error, but if the drying time a t 110" C. is extended to larger time intervals the error becomes appreciable. Accordingly, stability tests were applied at various temperatures to re-establish a correct drying temperatuie (Table I). By examination of the data of Table I the highest determined permissible drying temperature was found to be 87" C. At 99" there is a small but progressively cumulative loss in weight. Sample 1 was dried as received. Samples 2 and 3 were dried a t 79" preliminary to the drying experiments recorded in Table I. The samples employed in all the remaining analyses in this work were dried for 4 to 6 hours a t 80" to 85" C., followed by storage i n desiccators over Anhvdrone. OUTLINE OF ANALYTICAL PROCEDURES
A detailed description of the titratioaal methods employed for the evaluation of the amnioiiium nitratocerate is given in (f?, Chapter 4). Method A. Weighed samples of ammonium hexanitratrocerate were dissolved in perchloric acid and titrated, using a solution of accurately known concentration of sodium oxalate dissolved in perchloric acid. For these titrations nitroferroin f5-nitro-1,lOphenanthroline ferrous sulfate) served as indicator. Method B. Weighed samples of the same salt were dissolved under the same conditions and titrated, using a solution of accurately known concentration of arsenious acid made from known weights of Bureau of Standards arsenious oxide, also in perchloric acid solution. For this titration a drop of 0.01 M osmic acid in 0.1 A' sulfuric acid solution was employed as catalyst. Sitroferroin again served as indicator. Method C. Weighed samples of the same material were diswlved 111a small amount of water and added to a twofold excess of dilute ammonium hydroxide in the apparatus shown in Figure 1. The ceric hydroxide thus quantitatively precipitated was filtered u i d washed with five or six portions of water to remove ammonium nitrate. Finally, the washing ceric hydroxide was dissolved in hot 2 F sulfuric acid and the contents of the reaction veqscl were withdrawn into a 300-ml. filter flask. The reaction flask and filter disk were thoroughly washed with additions of sulfuric acid and the sulfatoceric acid thus obtained was titrated using standard arsenite prepared as for Method B, with osmic acid as catalyst but vith ferroin in place of nitroferroin as indicator [There are two wlfatoceric acids known to exist, having
V O L U M E 21, N O . 10, O C T O B E R 1 9 4 9
1235
the formulas H2Ce(SOa)a and HlCe(S04)r. The ammonium salt of formula (hH4),Ce(S04)1 may be precipitated from solutions of the former by the addition of ammonium sulfate to solutions of the acid which have only sufficient free sulfuric acid present to prevent hydrolysis. The latter sulfatoceric acid, when present with higher concentrations of sulfuric acid, upon similar treatment forms (~\"4)aCe(SOa)4.2Hz0. This tetraammonium sulfatocerate is often described as a double salt, 2(KH4)~SOa.Ce(S04)2.2H20, and is known as Coffelt's salt.] Thra reactions involved are as follows: Method A
+
2('ts(y()~)6--
+
('20a--
f [HClO.i] 2('c.+++ 2c02
+
+ 12x03-
-~
+
Table 11. Preparation of 4rsenite and Oxalate Solutions for Evaluating Purity of 4mmonium Hexanitratocerate
NO^)^-[(('iLH7S303)8Fe]+i [HClOal = C I ~ + + - [(ClLH7K;aOs)aFe]i'-$6XO;a-
+
Solution No.
Method B
+
+
+ + +
2('e(NO3)s-.kS03-~[HClOa] [Osmic Acid 1 H20 = 2Ce+CL'.iso4-- -2EI" 12s03-
1 2
(Indicator rtwtion thc. sani(~ as for JIrihod A )
6
+
~
+
acid. For weighings undcr 100 grams, a Troemner Kn. 10 balance was employed together with a calibrated set of stanrlard weights. The solution weights were taken using an August Sauter balance of 10-kg. capacity with a sensitivity of 10 my. or better. Calibrated weights were also employed in using this balance. Buoyancy rorrcctions Rere applied in all n-t4ghing\. The standard solutions thus prepared were used to titrate weigherl portions of the nitratoceratc; in all titrations weight burets of design previously described (3) were employed, Weight huret readings were determined after applying buoyancy corrrc.tions using the Troemner balanw.
3 4 5
PREPARATION OF PRIMARY STANDARD ARSENITE AND OXALATE SOLUTIONS
IVeighed quantities of sodium oxalate (Sational Bureau of Staiidards No. 40e) were dissolved in 2 F perchloric acid, transferred to a weighed 300- or 600-ml. glass-stoppered flask, and di1utc.d to approsimatcly 300 to 600 ml., employing 2 F perchloric
Typr
Arsenite Arsenite Oxalate Arsenite Oxalate Arsenite
Ic'a2Cz04 or AstOt
TaAen Grams 3.0019(5) 3.0103(0) 3.2335(0) 2.2845(0) l.5409(5) 2.2818(5)
Soliltion Weight Grams
1016.72(9) 1055,50(6) 548,3716) ,516.32(1) 278.58(4) 601.34(1)
icid Used
2FHC104 1 F HzSOa 2 F HClOI 2 FHClOl 2FHClOa 1 FH?SO4
Calrd Weight Value of of Salt (NHhCe(V0-)e G /a G lo 0.0028274 0.031343 0.0028520 0.031611 0,0060606 0,0494AO 0,0044245 0,049068 0.0055313 0.045239 0.0038112 0.046231
Sodium oxalate or arseiiious oxide (equivalent weights roughly 67 and 49.5, respectively), because of their low values, niako preferable the titration of weighed samples of ammonium hexanitratocerate by oxalate or arsenite to the reverse t,itration. By use of 0.05 to 0.06 N strengt,h of titrating solution a 40-gram sample reacts with 1.1 to 1.3 grams of ammonium hesanitratocerate. Sucah samples, individually weighed, afford better accuracy than the weighing of 0.2 t'o 0.25 gram of arsenious oxide or sodium oxalate which would be required for individual titrations by the reverse procedure. An added advantage, following the proredure as chosen, consists in thr fact that solufions of ammonium nitratorernte acidified using perchloric acid ch:uige titer detectably in 72 to 96 houm, whereas the solutions of in= senite or oxalate containing perchloric or sulfuric acid are stable indefinitely. The potentiometric titrational charactcristic- of thwe reactions have heen drscrihed ( 9 , i;cct,ions3 and 4).
solution^ of i ) r [ ~ ( l ~ ~ t ( ~ i isirtwgth i i i ~ i ~ , ~it1 i arsenious ositlc \vt:i'o prepared from A-ational Hurcau of Standards primary standard sample 83a with aritlificatioii using either 2 F perchloric acid or 1 F sulfuric2 acid as solvcnt. The wighcd portions wcrc p l a c ~ d in 250-nil. beakers and tiisrdvcd in a s n d l volume of wai,c'r by the addition of sufficicnt pc*llvtsndiuni hydroxide to form sotlinni arsmitr. Thest. saniplrs \ W I Y ~thc~iiacidified with acids of tho proper strength anti t raiidt5rrtd t o 500- or 1000-ml. glass-ntoppered flasks :and thra rollit ioii.< n - c ' i ~ ' wcighcd as prcviously islescribed.
_--_ -
--\ \
The data governing the preparatiori of ihr nrseriit,eand ox:+l:ite solutions are given in T&Ie 11. IlETER.MIN.ATION OF PURITY OF AMMONIUM HEXAXITRATOCERATE SAMPLE 1
Figure 1. Apparatus for Precipitation and Filtration
Sample 1 of ammonium hesanitratocerate was analyzed by solution in perchloric acid and titration, using standard of reference arsenious oxide and sodium oxalate as two individual and distinctive procedures. The third, and radically altered, procedure followed the scheme of the precipitation of Ce(1V) from excess ammonia solution, filtration, washing, solution in sulfuric acid, and final titration by a sulfuric acid solution of sodium arsenite prepared from primary standard arsenious oxide. Standard solutions 1, 2, and 3 (prepared as shown in Table 11) were eniployed for these analyses. Thc results of the first two procednics agree to a very close tolerance (by the two directly applied analyses). The results of the indirect ceric hydroxide isolation, solution, and titration [an indirect proredure involving isohtion of Ce(1V)] agree satisfactori1)- n-ith the direct titrations and serve to "niakr aosur:inre douhly sure." This indirect evaluation
1?36
ANALYTICAL CHEMISTRY Table 111. Determination of Purity of Ammonium Hexanitratocerate Sample 1 ! B n i g l o ~ing AszOa and Na?C?Oa as primary standards)
( K l i d z C e ( h O ~ i a Reference Sample Solution Weight (Table 11) Grams
Weight of Standard Soln Required
(NHdzCe(S0z)s Indicated Grams
Error of Recoi erq ’VI Q .
1,3022(4) 1.3038(7) 1.3055(4) 1,2042(2)
-0.19 -0.11 +0.01 -0.16
20.297(5)
1.0065(5) l.OOSO(8) 1.0060(2) 1.0271(7) 1.0092(1) 1.0050(6)
- 0 13 +0 . 4 0 - 0.05
Grams
1.3024(3) 1.3039(8) 1.3055(3) 1.3043(8)
1 1 1
1
41.546 41.597(5) 41.651 4 1 . GO9
1.0066(8) 1,0076(8) 1,0050 7) 1,0270[0) 1.0093(8) 1.0051(7)
3 3 3 3 3 3
20.328(5) 20.266(5) 20.713(6) 20.351 20.267(5)
Cni( d
Salt
Piintv
‘7
09.97(!0 99.98(4!
100.00
99 9813)
99.98(7)
f0.17 -0.17 -0
11
4v. 1.3016(1) 1.3035 1) 1.3048{1) 1.3043(1)
2 2 2 2
41.164 41.229 41,262 41 252
1.3012(4)
1 3032(9)
1.304(4) 1 3040(1)
-0.37 -0.22 -0.47 -0.50
100.03(7) 9 9 , !)I) ( 3 I 100.01(6i 99.98(3) 99.08(9) 100.00(1) 99.97(3) 99.98(8) 99.96(6) 99.97(1) 99.97(3)
oxide and 99.96% for sodium oxalate sample 40e were applied to the calculations of Table 111. Solution densities for buoyancy correction were calculated from weight data and the specific gravity of the acids used in their preparation. The procedures described in the analysis of sample 1 of ammonium hexanitratocerate were repeated in the case of sample 2 with results shown in Table IV. By examination of Table IV, the same conclusions may be drawn as those observed with regard to Table 111. The most probable value for the purity of ammonium nitratoceratr sample 2 is 99.98%. ERRORS IN CONVERSION OF AMMONIUM HEXANITRATOCERATE TO PERCHLORATOCERIC ACID AND CERIC HYDROXIDES TO SULFATOCERIC AClD
Solutions of ammonium hexanitratocerate in 1F perchloric acid have an oxidation potential of 1.71 volts; they are not permanently stable and very Table 17’. Determination of Purity of Ammonium Hexanitratocerate gradually diminish in oxidation value with time Sample 2 of storage. If stored a t ice box temperatures and (Employing IsgOa and KazCtOl a5 primary standards) in dark bottles, solutions (even of 0.001 N conWeight of centrations) need be restandardized only a t 72- to (NHl)zCe(NOa)a Reference Standard Error Calcd. Sample Solution 96hour intervals (1). For the preparation of of Sa!t Soln. (NH4)zCe(KOa)6 Weight (Table 11) Reauired Indicated Recoverv Puritv 0.05 to 0.1 N solutions of ammonium hexanitratoGrams Grams Grams Mg. % cerate in perchloric acid for use in macrovolu1.0039 3) 4 20.345(5) 1.0038(5) -0.08 99.99(2) 1.0022 3) 20.307 metric determinations, restandardization is re1.0019(6) -0.27 99.97(3) 1.0076 8) 20.397 1.0063(8) -0.40 99.96(0) quired only after 72 hours’ storage ( 2 ) . Storage 1.0063(8) 1.0021 3) 4 20.311 1.0021(3) *o.oo 100.00 20.395 1.0062(8) -0.01 99.99 in dark bottles in a cool place is recommended. 1.0074(3) 4 20.406(5) 1.0068(6) -0.50 99.97(9) In making perchloric acid solutions of amAv. 99.97(7) monium hexanitratocerate, great care should be 22,140 1.0022(1) 5 1 0020 2) -0.19 99.98(1) taken to avoid the presence of organic matter. 5 22.189 1: 004215) -0.28 99.97(1) 22.541 5 1.0201(8) -0.27 99.97(4) The importance of pure water and thoroughly 22.387 5 1.0132(1) 1.0133(3) -0.12 99.98(8) clean containers cannot be overemphasized. Av. 99.97(9) Cellulose, sugars, alcohols, and many other or26.251(5) 1.1091(5) -0.26 99.97(6) ganic materials are readily and quantitatively 25.539 1.0790(4) -0.49 99.95(5) 25.711 (5) 1.0863(4) -0.43 99.96(1) oxidized by the cerate ion in perchloric acid 24.841(5) 1,0495( 1j -0.15 99.99(1) 24.561(5) 1.0377(4) -0.22 solution ( 2 ) . The use of too much stopcock 99.97(8) 25 .662 1.0842(3) -0.38 99.96(5) grease in burets may introduce errors. The presA v . 99.96(9) ence of carbonaceous dusts causes perchloratoAv. of 16 individual analyses, 99.97(7)% cerate solutions to change titer. Contact with platinum in the form of sponge or platinum black causes comparatively rapid reduction of such ,of the oxidation equivalent of the hexanitratocerate proves solutions. These are all penalties associated with an oxidation conclusively that its total oxidation value lies in its cerium conmedium of exceptionally high oxidation potential. tent. In addition, the procedure as described is shown to be a Special directions must be followed in the preparation of solureliable scheme for the preparation of solutions of sulfatoceric tions of ammonium hexanitratocerate in perchloric acid. acid [H4Ce(S0,),] of predetermined titer without the ammonium To pre are such a solution of 0.1 N strength, 54.826 grams nitrate and nitric acid which would be present if such solutions (correctel weight of the ure salt) are placed in a 1000-ml. were to be prepared by direct solution of ammonium hexanitratobeaker and 83.8 ml. of 72gperchloric acid are added. The salt cerate in sulfuric acid and dilution to a fixed volume. The data is not soluble in perchloric acid of this strength, but should be stirred with it for 2 minutes. A 100-ml. portion of water is are given in Table 111. Buoyancy corrections were calculated added and the stirring is continued for 2 minutes. The addition using the determined value of 2.61 as the specific gravity of of 100 ml. of water is repeated a third, fourth, and fifth time with ammonium hexanitratocerate a t 25 O C. intermediate stirring for 2-minute intervals. By this necessary By examination of Table I11 it is observed that the maximum procedure all the ammonium hexanitratocerate will be in solution a t 5UO- to 600-ml. volume and may be transferred to a 1000average deviation in results among the three methods IS 2 parts ml. graduated flask and diluted to volume. The ammonium in 10,000. The average analysis in 14 determinations indicates hexanitratocerate would dissolve completely to form a crystalthat the purity of this sample of hexanitratocerate is 99.99%. clear solution if it were added to the perchloric acid after dilution The indirect procedure of analysis, following isolation of this with mater to 500 to 600 ml. Such a solution after 24 hours salt’s cerium content, shows results 0.01 % lower-namely, would be partially precipitated, owing to the presence of complex nitratoperchloratocerates which are sparingly soluble. If the 99.98%. This lower value may be attributed to the introduction salt crystals are added to 72% perchloric acid the Ce(NOa)e-of additional manipulative procesw of a less direct nature, complex ion is sufficiently completely converted to the Cefollowing which the values were dcteimined. The results given in (ClO4)e-- ion to prevent the formation of insoluble material. Table I11 are consecutive :iiiJyses with no omission in any given The substitution of sulfuric acid for perchloric acid requires system of analyses. The recommended purity given with the BUthat a duplicate solution procedure be applied d t h slow addition reau of Standards certificates of 99.99% for standard 83a arspnious of the first 100 ml. of water over a 5-minute period with stirring Av. of 14 individual analyses, 99.88(9)70purity
I
::E135l
i
Av.
1237
V O L U M E 2 1 , NO. 10, O C T O B E R 1 9 4 9 (56 ml. of 95% sulfuric required in place of 83.8 ml. of 72% perchloric acid). Mixed nitratosulfatocerates are more insoluble than the mixed nitratoperchloratocerates and would precipitate copiously upon standing were this procedure not followed, and perchloratoceric Solutions of sulfatoceric acid, H4Ce( acid, H2Ce(CI04)5, prepared by the process described above, contain aninionium and nitrate ions. The solutions are of known normality and for moat applications serve admirably ( 8 ) . If ammonium arid nitrate ions are undesirable, solutions of perchloratoceric acid arc best prepared by the electro-oxidation of perchloric acid (9, pages 5 to 9). For the preparation of sulfatoceric acid the method described above in connection with the detci mination of the purity of ammonium hexanitratocerate is employed. The apparatus in Figure 1 serves conveniently for the precipitation of ceric hydroxide, with removal of the ammonium and nitrate ions, solution of the ceric hydroxide in warm 2 F to 4 F sulfuric acid, and final dilution to the predetermined volume. .41i Ace sintered-glass filtering crucible with porosity E may be used to filter and wash ceric hydroxide. Filter paper or a platinum hiuriroe filtering crucible must be avoided. If filtei papei is used some of the sulfatoceric acid is invariably reduced; results m ‘ ~ ybe 1 to 2?4 low. Low results are also caused by contact between hot sulfatoccric acid and platinurn sponge if a Munroe crucible is emplo) ed DECOMPOSITION OF PERCHLORQTOCERIC ACID SOLUTIONS IN CONTACT WITH PLATINUM SPONGE
A solution of approximately 0.1 S perchloratoceric acid was prepared from ammonium hexanitratoceratc and perchloric acid, and was stored in contact with platinum sponge formed by the ignition of 1 gram of chloroplatinic acid. The perchloratoceric acid solution was stirred continuously a t ordinary temperatures and analyzed a t stated intervals. The original normality was 0.1020. After 2 hours it was 0.0996, after 5 hours 0.0980, after 22 hours 0.0925, and after 96 hours 0.0775 N . This is a drop of almost 25% in oxidation value. Sulfatocerate solutions are not as extensively affected, but the loss is readilv noted when hot solutions are filtered using a platinum Munroe filtering crucible. Graphitic carbon is known to evert a similar catalytic reducing effect. -
Table V.
Analysis of Sample 1 after More Than 2 Years’ Storage under Ordinary Conditions
[Test of stability a n d retentiou of oxidation value with long-term storage. As203 solution, 0.027415 P a m per gram, equivalent to 0.303935 p m m of (NH&Ce(XOa)s per gram of solution] Sample so.
1
,
3
1 5 6
.48zOa Soln.
Drams 10.3884(6) 8.5872(4) S.1556(2) 8.3569(5) 7,8023(9) 8.9750(8)
(PI”dzCe(N03) 6 Taken
Found
Grams
Grams
%
3.1578(4) 2 . 6 1 01( 0 ) 2.4788(4) 2.5402(5) 2.3719(6) 2.7281(2)
3.1673(5) 2.6099(6) 2.4787(8) 2.5399(7) 2.3714(2) 2.7278(9)
O R . 086
Purity
9t,.994 99.997 99.989
Av.
99,977 99,990 99.988
STABILITY O F CRYSTALLINE AM.\IOI\IUiM h l T R 4 T E WITH LONG PERIODS O F STORAGE
Ammonium nitiatocerate consists of oxidizable cation-, 2NH4+,and a complex anion, Cc(N03)6-, vhich is known to be a powerful oxidizing material. Solutions of hexanitratoceric acid, H,Ce(NOs)e, in nitric acid are known to be unstable t o a minute but measurable extent. This instability is brought about by the decomposition of water with the evolution of oxygen through the intermediate liberation of free radical hydroxyl groups. In order to study the shelf life of crystalline ammonium nitratocerate, after completion of the present work and preparation of the manuscript it was thought appropriate to delay 2 years to permit n shelf life test period and subsequent additional
P
analysis. The procedure duplicated that previously described and employed the same corrections and the same drying temperature Sample reagent 1 with 2 years and over “shelf existence” was reanalyzed with the results shown in Table V. Table V shows that the new primary standard, (NH&Ce(N03)5,does not measurably alter in value as an oxidant with wtended periods of storage. SiJU MARY
Ammonium hexanitratocerate, proposed a a primary standard in oxidimetry, has been reinvestigated. This salt is commercially available in highly pure form for use as a precision primary st,andard. The determination of a s:ttisfaetory drying temperature to eliminate adsorbed moisture or nitric acid is shown to be 87” C. in place of the previously recommended temperature of 110”. Occluded moisture is absent from cryst,alline ammonium nitratucerate, (NH4)pCe(XOs)e. Ammonium hexanitratocerate is shown to possess the ten most desirable chemical and physical properties governing the selection of a material to be used as a primary standard. Simple qualitative tests are described for use irr rapid preliminary testing of samples of animonium hexanitr:itoKO commercially available samples of ammonium hcxanitratocerate were standardized by comparison with National Bureau of Standards arsenious oxide (primary standard 83a) and sodium oxalate (primary standard 40e), and their purities were shown to be 99.99 and 99.98%. Their cerium content was isolated as ceric hydroxide and converted to sulfatoceric acid, H&e( SO4)>. Their purity R&Sthus determined by the use of reference standard arsenious oxide. By this indirect procedure their oxidizing value was shown to be entirely due to the cerium content. The results thus obtained agree with the previous results within 1 part in 10,000. In the analyses reported all appropriate corrections were applied. The weights were calibrated by comparison with a. master set which had been evaluated a t the Bureau of Standards. A Troemner Xo. 10 balance accurate to *0.05 mg. by direct. observation and to *0.02 mg. by estimation was employed for all weighings of less than 100 grams. -4 special August Sauter balance of 10-kg. capacity accurate to *IO mg. was employed for weighing solutions of 250 to 1000 grams, Weight burets were employed for all titrations with corrections for buoyancy applied to all weighing,.. The weights of arsenious oxide, sodium oxalate, and ammoriiuni hexanitratocerate were likewise corrected to the vacuum basis. Direct’ions are given for the preparation of sulfatoceric acid from ammonium hexanitratoccrate in the form of solutions of predetermined normality. 1G1on-n amounts of ceric hydroxide prepared from weighed amounts of the new primary standard, olved in hot dilute sulfuric acid, and diluted to a predetered volume. This process is equivalent to the ability to use a pure sample of ceric oxide as a primary standard. Such samples of pure ceric oxide are not soluble in sulfuric acid. The shelf life of the new primary standard is adcquate.
LITERATURE CITED
(1) Salomon,Gabrio, and Smith, Arch. Biochem., 11,433 (1936). (2) Smith, G . F., “Cerate Oxidimetry,” Columbus. Ohio. G. Frederick Smith Chemical Co., 1942. (3) Smith, G. F., Ind.Eng. Cham., 18,1216(1926). (4) Smith and Richter, “Phenanthroline and Substituted Phenanthroline Indicators,” Columbus, Ohio, G. Frederick Smith
Chemical Co., 1944. (5) Smith, Sullivan, and Frank, IND. ENG.CHEM., - 4 s . 4 ~P:D., . 8,449 (1936).
RECEIVED November 8,1947.
